Class 12 CBSE Chapter 2 Sexual Reproduction in flowering plants

SEXUAL REPRODUCTION IN FLOWERING PLANTS

INTRODUCTION :

The process of reproduction in plants involves a specialized structure known as a flower. In a typical or complete flower, there are four main parts: the calyx of sepals, the corolla of petals, the androecium of stamens, and the gynoecium of carpels. While the calyx and corolla are considered accessory whorls of the flower because they do not directly participate in the process of reproduction, they do provide support. On the other hand, the androecium and gynoecium are essential whorls since they are directly involved in the process of reproduction.

The male sex organ of a flower was first identified by N. Grew, who discovered that it was the stamen (a key aspect of plant anatomy).
Jacob Camerarius is credited with reporting on the concept of plant sexuality, having identified the anthers as male sex organs and the ovary, style, and stigma as female sex organs. He also recognized that the interaction between the two sex organs is essential for seed formation.
The importance of pollination and the role of insects in this process were first recognized by Josheph Kolreuter.
C.F. Wolf is known as the Father of plant embryology.
Prof. P. Maheshwari is considered the Father of Indian plant embryology, having authored a book titled “An Introduction to Embryology of Angiosperms.”

SEXUAL REPRODUCTION

Sexual reproduction refers to the process by which new organisms are developed through the formation and fusion of gametes. In flowering plants, stamens serve as the male reproductive organs while carpels are the female reproductive organs.
In angiosperms, male and female gametes are formed through meiosis in their respective sex organs. The gametes then fuse together to form a diploid zygote, which ultimately develops into an embryo.
The process by which the embryo is formed through meiosis and fertilization is known as amphimixis.
The study of sexual reproduction in flowering plants includes both pre-fertilization and post-fertilization events and structures.
Pre-fertilization events include the male reproductive organ (androecium), female reproductive organ (gynoecium), pollination, and fertilization.
Post-fertilization events include the development of endosperms, embryo development in both dicot and monocot plants, and the formation of seeds.

The male reproductive organ

The male reproductive organ in flowering plants is known as the androecium, and its unit is called the stamen.
The stamen, also known as a microsporophyll, typically consists of three parts: a long, thin structure called the filament that joins the stamen to the thalamus, a swollen structure called the anther that bears spores, and a small region called the connective that contains vascular tissues and attaches the anther and filament.
The anther is generally a bilobed structure, meaning that it has two anther lobes and is referred to as dithecous.
Each lobe of the anther contains two chambers, called pollen sacs or pollen chambers. Thus, a typical anther has four pollen sacs, making it tetrasporangiate.
Pollen grains are formed inside the pollen sacs through the meiotic division of pollen mother cells.
At maturity, the sterile tissue present between the pollen sacs degenerates, causing the two pollen sacs in each anther lobe to fuse together. As a result, only one chamber is visible in each anther lobe at maturity, and a mature anther has two chambers at the time of dehiscence.
In some plants, such as those in the Malvaceae family, the stamen has only one anther lobe, making it monothecous, and it contains only two pollen sacs, known as bisporangiate.
Monothecous anthers can also be found in Moringa and Wolffia plants.
When there is only one microsporangium per anther, as in the case of Arceuthobium, this condition is called monosporangiate.

Click here for Part-2 Female reproductive whorl

Click here for Part-3 Pollination and Fertilization

Click here for Part-4 Fertilization Endosperm and Embryo Development, seed,fruit

Click Here for NCERT solution Chapter 2 Sexual Reproduction in Flowering Plants

ANTHER STRUCTURE

The anther develops from more than one archesporial cell, making it of Eusporangiate origin. In a transverse section, the anther appears almost spherical.

Fig.: Transverse section of a mature dithecous anther.

An anther is composed of several layers of walls, including the epidermis, endothecium, middle layer, and tapetum.

EPIDERMIS The epidermis is the outermost protective layer of the anther. It is a continuous layer, but it is not derived from archesporial cells. In the smallest parasitic angiosperm, Arceuthobium, the epidermis has fibrous thickening, making it known as the exothecium.

ENDOTHECIUM This layer is situated below the epidermis. It is a single-celled thick layer that undergoes various changes during anther maturation. The outer wall of the endothecial cells remains thin-walled, while the inner walls and radial walls become thick due to the thickening of α-cellulose fibers. Callose bands are also present along the radial walls, but they are absent in some areas known as stomia, which is where anthers dehisce. The presence of fibrous thickening in the endothecium makes it hygroscopic, thus aiding in anther dehiscence.

MIDDLE LAYER The middle layer of the anther consists of parenchymatous cells and is a one to three celled thick structure. It functions in food storage. However, it is ephemeral in nature and is absent in a mature anther.

TAPETUM The tapetum is the innermost thick layer of the anther and acts as a nutritive layer. It surrounds the pollen sacs and is a single-celled layer. The cells of the tapetum are initially diploid but become polyploid through endomitosis, resulting in cells with many chromosomes.

The tapetum plays an important role in meiotic cell division in microsporogenous cells and in pollen development. It absorbs food from the middle layer and provides nutrition to the microspore mother cells or microspores. Additionally, the cells of the tapetum secrete hormones and enzymes such as callase.

There are two types of tapetum: amoeboid and secretory.

Amoeboid Tapetum/Invasive tapetum/Periplasmodial tapetum: This type of tapetum is found in primitive angiosperms. It absorbs all nutrients from the middle layer, which then degenerates. The tapetal cells convert the absorbed nutrients into special granules called protoplast bodies. The innermost layer of tapetum dissolves and releases its protoplasts into the microsporangium cavity, where they are known as periplasmodium. Microspore mother cells are surrounded by periplasmodium, which provides nourishment to the developing microspores. After degeneration, this type of tapetum provides nutrition to the microspores. Examples of plants with amoeboid tapetum include Typha, Alisma, and Tradescantia.

Glandular or Secretory Tapetum: This type of tapetum does not degenerate quickly. Cells of this tapetum remain intact throughout microspore development. It absorbs nutrients from the middle layer and secretes them into the pollen sac cavity without storing them. Most flowering plants, such as Capsella, have this type of tapetum. Before degeneration, the tapetal cells form special granules called Pro Ubisch bodies in the cytoplasm. Pro Ubisch bodies are surrounded by sporopollenin and become Ubisch bodies or orbicules. After the tapetum degenerates, Ubisch bodies are released into the pollen sacs and participate in the formation of the exine of microspores. Generally, sporopollenin participates in the formation of the outer covering (exine) of pollen grains.

Click here for Part-2 Female reproductive whorl

Click here for Part-3 Pollination and Fertilization

Click here for Part-4 Fertilization Endosperm and Embryo Development, seed,fruit

MICROSPOROGENESIS

Microsporogenesis is the process of formation of microspores or pollen inside the anther. Initially, the anther appears as an outgrowth-like structure that is spherical or oval in shape. It consists of undifferentiated and homologous meristematic cells surrounded by a single-cell-thick outer layer called the epidermis. Vascular tissue is formed in the middle region and, simultaneously, four cells located just below the epidermis in vertical rows in the region of hypodermis at the four corners become large. These cells have a visible nucleus with dense cytoplasm and are called archesporial cells.

Archesporial cells divide periclinal to form primary parietal cells below the epidermis and primary sporogenous cells towards the center. Both of these cells usually undergo further divisions to form the complete structure of the anther except the epidermis. The primary parietal cells undergo further periclinal and anticlinal divisions to form a series of 3-5 layers making up the walls of the anther.

The primary sporogenous cells divide by mitotic division to form sporogenous cells, which later differentiate into microspore mother cells (MMC) during the formation of the wall of the pollen sac. Each MMC divides to form four haploid microspores or pollen grains by meiotic division or reduction division. Initially, all four microspores are attached together with the help of a callose layer, and this group of microspores is called a tetrad. After some time, this callose layer dissolves by callase enzyme, which is secreted by tapetum.

Normally, each microspore mother cell can form a tetrad by meiotic division. But in some plants like Zostera, some microspore mother cells become sterile and provide nutrition to the remaining microspore mother cells. Similarly, tapetum is not well developed in the Gentianaceae family, so some cells of sporogenous tissue become sterile and provide nutrition to the remaining sporogenous cells.

Types of tetrads: The arrangement of microspores in tetrad condition varies and includes the following types:

Tetrahedral tetrad (most common): Four haploid microspores arranged in a tetrahedral form. E.g., Capsella in Dicotyledons.
Isobilateral tetrad: This condition is found in monocotyledons. Microspores are arranged at the lateral side of each other.
Decussate tetrad type: In this type, two microspores are positioned at right angles to the other two microspores. Example: Magnolia.
T-shaped tetrad: In this type, two microspores are aligned longitudinally and two microspores are aligned transversely. Examples: Aristolochia and Butomopsis.
Linear tetrad: In this type, all four pollen grains are arranged in a linear sequence. Example: Halophylla and Halophila.

All of the above types of tetrads can be found in Aristolochia elegans.

The pollen grain is the initial cell of the male gametophyte. It is surrounded by two distinct layers: the outer layer (wall) called exine, which is thick, rigid, and ornamented, and the inner layer called intine, which is thin, soft, and elastic. Exine is formed by cutin and sporopollenin, a highly resistant material that is absent in the pollen grains of Zostera. The presence of sporopollenin ensures that fossils of pollen grains are always found in good condition. The sculpturing or designs on the surface of pollen grains are provided by tectum. Intine is made up of pectin and cellulose or pecto-cellulose. Germ pores, which are areas of absent or thin exine, are present on pollen grains. During pollen grain germination, intine comes out through a germ pore in the form of a pollen tube. The number, structure, and ornamentation of germ pores are significant features for taxonomy. Palynology is the study of pollen grains. Capsella pollen grains exhibit the tricolpate type of germ pore, characterized by three colpus or slits. Monocots have only one germ pore, and their pollen grains are called monocolpate.

SOME FACTS ABOUT POLLEN :

Translator apparatus: In Asclepiadaceae (Calotropis) and Orchidaceae family, the pollen grains join together to form bag-like “Pollinium”. The Pollinium of Calotropis is a constituent of the “Translator apparatus”, which consists of paired pollinia, Gynostegium, Corpusculum (adhesive, bilobed disc), Pollinium, Caudicle (Retinaculi), United style A, C, B, and the Translator apparatus.
Aero-allergens: Pollen grains of many species present in the air cause allergies and bronchial afflictions, and are called “aeroallergens”. Examples include Chenopodium, Parthenium (carrot grass), Sorghum, and Amaranthus. Hay fever is caused by pollens of Ambrosia. In some people, allergic pollens cause chronic respiratory disorders such as asthma and bronchitis. Parthenium, which came into India as a contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy.
Pollen tablets and syrups: Pollen grains are rich in nutrients. It has become a fashion in recent years to use pollen tablets as food supplements. In western countries, a large number of pollen products in the form of tablets and syrups are available in the market. Pollen consumption has been claimed to increase the performance of athletes and racehorses.
In the Cyperaceae family, only one functional pollen grain is formed from a pollen mother cell, e.g., Cyperus.
Largest pollen: Mirabilis.
Smallest pollen: Myosotis.
Longest pollen: Zostera (sea grasses), Filiform pollen or filamentous pollen grain. The pollen grain is long, ribbon-like, without exine.
Viability of pollen grains: In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release, and in some members of Rosaceae, Leguminoseae, and Solanaceae, they maintain viability for months. The period for which pollen grains remain viable is highly variable and depends, to some extent, on the prevailing temperature and humidity.
Pollen banks: It is possible to store pollen grains of a large number of species for years in liquid nitrogen (-196°C). Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programs.

Click Here for NCERT solution Chapter 2 Sexual Reproduction in Flowering Plants

Structure of pollen Grain

Pollen grains are typically spherical and have a diameter of about 25-50 micrometers.
They have a prominent two-layered wall. The outer layer, called the exine, is thick, rigid, and ornamented. It is mainly composed of sporopollenin, one of the most resistant organic materials known, which can withstand high temperatures, strong acids, and alkalis. No enzyme that degrades sporopollenin is known.
The presence of sporopollenin allows pollen grains to be well-preserved as fossils, which can help predict the presence of natural resources like petroleum and coal in the earth.
The inner wall of the pollen grain, called the intine, is thin, continuous, soft, and elastic in nature. It is made up of pectin and cellulose or pecto-cellulose.
The exine may be absent or present as a thin layer in some areas, called germ pores. The intine comes out through one of these pores during the germination of the pollen grain in the form of a pollen tube.
The exine displays a fascinating array of patterns and designs, and the number of germ pores, structure, and ornamentation of the exine is a significant feature of taxonomy.
The study of pollen grains is called palynology.
Most dicots have pollen grains with three germ pores, called tricolpate pollen grains, while monocots have pollen grains with a single germ pore, called monocolpate pollen grains.
In plants where pollination occurs via insects, the pollen grains have an oily layer called pollen-kitt, which is composed of lipids or carotenoids. Capsella is an example of a plant with pollen-kitt.
The functions of pollen-kitt include protecting the pollen grain from harmful ultraviolet rays, helping it stick to insects, and attracting insects with its yellow color.

lick here for Part-2 Female reproductive whorl

Click here for Part-3 Pollination and Fertilization

Click here for Part-4 Fertilization Endosperm and Embryo Development, seed,fruit

ANTHER DEHISCENCE:

Anther undergoes various changes during its maturation. l Initially, the middle layer degenerates as tapetum absorbs the food.

The connective (vascular tissue) and epidermis are also present in a mature anther as the outer covering.

Sterile tissues between the pollen sacs degenerate and fuse the two pollen sacs of each anther lobe together.

Thus, a mature anther contains only two pollen sacs. l Anther dehiscence usually occurs during the dry season when the water is lost from endothecial cells.

The outer walls of endothecial cells contract and become concave, while inner and radial walls have fibrous thickening and do not contract.

The contraction of the outer walls causes tension over the entire surface of anther, leading to the breaking of thin-walled stomial cells.

This breaking results in anther dehiscence, and the pollen grains present in the pollen sacs are released into the atmosphere.

Most Angiosperms show longitudinal anther dehiscence, including Capsella.

Click Here for NCERT solution Chapter 2 Sexual Reproduction in Flowering Plants

MICRO-GAMETOGENESIS OR DEVELOPMENT OF MALE GAMETOPHYTE

The first cell of the male gametophyte in flowering plants is the microspore or pollen grain, which begins to partially germinate or develop before the anther dehisces (pre-pollination development). This development takes place inside the pollen sac of the anther, which is known as in-situ development. The stages of a microspore maturing into a pollen grain include the division of the nucleus of the pollen grain by unequal mitotic division, resulting in the formation of two unequal-sized nuclei: the generative nucleus and the tube nucleus or vegetative nucleus. After dense cytoplasm surrounds both nuclei, unequal cytokinesis occurs, resulting in the formation of two cells of unequal size: the larger vegetative cell or tube cell containing the irregular-shaped tube nucleus and the smaller generative cell containing the small nucleus. The mature pollen grain contains two cells, the vegetative cell and the generative cell, and is considered a partially developed male gametophyte. The generative cell transforms into a vermiform or spindle-shaped structure with dense cytoplasm and detaches from the wall and enters inside the vegetative cell and floats in the cytoplasm of the vegetative cell. In over 60% of angiosperms, pollen grains are shed at the two-celled stage, while in the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (three-celled stage).

After pollination, further development of the pollen grain takes place on the stigma of the carpel (post-pollination development). Pollens absorb moisture and sugar content from the stigma, which increases the volume of cytoplasm and exerts pressure on both outer layers. Because of this pressure, intine comes out through any one germpore in the form of a tube-like structure called the pollen tube. The vegetative nucleus enters into the pollen tube and assumes the terminal (tip) position, and the spindle-shaped generative cell enters the pollen tube. Inside the pollen tube, the generative cell divides mitotically to form two non-motile male gametes. The mature male gametophyte of angiosperms is a three-celled structure that includes one vegetative cell and two male gametes. The male gametophyte of angiosperms is highly reduced and completely depends on sporophyte. The pollen grain represents the male gametophyte. The formation of a mature pollen grain from a microspore mother cell or pollen mother cell requires one meiotic and one mitotic division, while the formation of a mature male gametophyte requires one meiotic and two mitotic divisions.

Click here for Part-2 Female reproductive whorl

Click here for Part-3 Pollination and Fertilization

Click here for Part-4 Fertilization Endosperm and Embryo Development, seed,fruit

Click Here for NCERT solution Chapter 2 Sexual Reproduction in Flowering Plants